Electrically erasable programmable read only memory (“EEPROM”) is a type of rewritable non-volatile (“NV”) memory chip that holds its content without power. EEPROMs have lifespans, measured in number of write cycles, considerably greater than electrical programmable read only memories (“EPROMs”), the technology that preceded EEPROMS.
EEPROMS use a floating gate to hold a charge. Based on whether charge is trapped on the floating gate, the EEPROM transistor acts like a permanently-open or closed transistor. Charging the floating gate is accomplished by grounding source and drain terminals of the EEPROM transistor and placing a voltage on a control gate (write terminal). Applying a reverse voltage to the control gate causes the charge to dissipate into the substrate.
SONOS (poly-Silicon-Nitride-Oxide-Silicon) is a type of non-volatile (“NV”) memory that has attracted much attention due to its advantages over traditional floating-gate flash. Some of these advantages include lower programming voltages, better scalability, and improved cycling endurance. However, SONOS memory still suffers from endurance and retention degradation over its lifespan due to operational stresses. These operational stresses arise from the programming and erasing voltages applied to the control gate and from elevated operating temperatures. Eventually operational stresses lead to trapped charge accumulation on the floating gate of the EEPROM, resulting in the inability of an EEPROM memory cell to retain data.